I am not really sure when mascots started to be a common face at football, but their popularity has risen in the last 20 years and now all clubs have a mascot. I do not remember them being very common when I first started to go to matches 40 years ago.
Mascots come in all shapes and sizes and generally greet fans before the game and then walk around the pitch to say hello to fans before the game starts, then at half-time and also at the end of the match.
The Bournemouth Mascot is called Cherry Bear and this is a recent photo (April 22, 2017) at the match against Middlesborough.